1. Field of the Invention
The present invention relates to a head driving device, a liquid-ejection head unit, and a liquid ejection apparatus.
2. Description of the Related Art
As an image forming apparatus, such as a printer, a facsimile, a copier, and a digital printing device, a liquid-ejection-recording image forming apparatus (for example, an inkjet recording apparatus) that uses a recording head constituted, for example, of a liquid ejection head (ink-droplet ejection head) that ejects in droplets has been known. This liquid-ejection-recording image forming apparatus forms a desired image by ejecting ink droplets from the recording head onto a recording medium (for example, a paper sheet).
The recording head is equipped with a nozzle that ejects ink droplets, an ink passage (pressure chamber) with which the nozzle communicates, and a pressure generating unit that applies pressure to ink inside the ink passage, and generally, a so-called piezoelectric type in which a piezoelectric device is used as a pressure generating unit, and by micro-vibrating a vibrating plate that forms a wall of the ink passage by the piezoelectric device, the volume inside the ink passage is changed to eject ink droplets; a so-called thermal type in which ink droplets are ejected by pressure caused by bubbles generated by heating ink inside the ink passage by using a heat element; and an electrostatic type in which a vibrating plate that forms a wall of the ink passage and an electrode are arranged opposing to each other, and by deforming the vibrating plate by static electricity generated between the vibrating plate and the electrode, the volume inside the ink passage is changed to eject ink droplets have been known.
It is explained with the piezoelectric type as an example. Multiple nozzles are formed in a recording head, and an ink passage (pressure chamber) and a pressure generating unit (the pressure generating unit is explained with an example of a piezoelectric member (piezoelectric device)) are provided for each nozzle. These nozzles are arranged in a predetermined direction (hereinafter, this direction is referred to as nozzle row direction).
All of the piezoelectric members are electrically connected in parallel between a common electric supply line and a ground wiring, and to each of the piezoelectric members, a switching device is electrically connected in series. A signal (driving waveform) is generated by a driving-waveform generating circuit, and is selectively distributed to the respective piezoelectric members through the power supply line and the switching device. That is, when a specific switching device is selected to be on based on print data, a driving waveform is applied to the piezoelectric member through the power supply line, and ink droplets are ejected from a specific nozzle corresponding to the piezoelectric member to which the driving waveform has been applied.
Moreover, there also is a recording head that ejects various kinds of ink droplets (for example, a large droplet, a medium droplet, and a small droplet) having different ink volumes to improve the gradation of an image by changing the size of dots that are formed on a recording medium. In such a recording head, ink droplets are successively ejected while changing the drop speed by using a driving waveform having multiple pulse trains within a printing cycle, and the driving waveform is configured so that the droplets coalesce into one droplet in the air. In this case, a common driving-circuit method in which one common driving waveform having various driving waveform elements to eject various kinds of ink droplets combined is used, and a necessary part of waveform is selectively applied to respective piezoelectric members by a switching device is generally used.
Furthermore, generally, this driving waveform requires a waveform of a comparatively large voltage amplitude such as 20 volts (V) to 40 V, and a driving-waveform generating circuit to generate and drive such a waveform is comparatively large scale, and the consumed power is also large. Therefore, it is not arranged in a recording head that is required to be in a small size, and a driving waveform that is generated by another circuit board is often provided to the recording head through a power supply line. Moreover, a switching device that is provided for each piezoelectric member is often integrated with a control unit that generates an on/off selection signal and arranged close to the piezoelectric member in the recording head.
This integrated switching device includes a transistor, and uses a high-voltage power metal oxide semiconductor field-effect transistor (MOSFET) and the like to drive a relatively large voltage amplitude is used, to be a large size. Therefore, the ratio to the size of the integrated circuit is also large.
To form a high-quality image in an inkjet recording apparatus, it is demanded to put a desired amount of ink droplet at a desired position on a recording medium. Therefore, a driving waveform provided to a piezoelectric member is appropriately configured considering the ink drop speed, the stability in the ejection state (curved ejection, satellite, a mist generating state), and the like.
As a technique of correcting variations in the droplet amount and in a landing position of ejected liquid, and of forming a high-quality image, for example, a technique disclosed in Japanese Unexamined Patent Application Publication No. 2001-301206 (Patent Literature 1), or in Japanese Unexamined Patent Application Publication No. 2009-241345 (Patent Literature 2) is publicly known.
In Patent Literature 1, it is described that to prevent instability of ejection and a change in an ink droplet volume caused by meniscus vibration of residual ink from previous ejection (hereinafter, “residual vibration”), a driving waveform of a dot is changed based on whether ejection is performed right before and right after the dot, and on a shape of the driving waveform of an ejection pulse signal.
Furthermore, in Patent Literature 2, it is described that considering not only an influence of ejection before and after, but also an influence caused by crosstalk in which energies generated at the time of ejection from adjacent nozzles propagate mutually, a type of ink droplet at the time of an arbitrary ejection timing of each nozzle is determined referring to an ejection history of the nozzle included in chronological information, and a type of ink droplet that is associated with an arbitrary ejection timing of a nozzle other than the nozzle.
However, in the techniques disclosed in Patent Literatures 1 and 2, changeable driving waveforms have increased by only one or more, and the accuracy has not been sufficiently high to correct the ejection speed and the ejection droplet amount, and to cause a desired droplet amount of ink to land at a desired position. Inversely, to make the correction accuracy sufficiently high, it is necessary to generate more driving waveforms, and to select a driving waveform from among them according to previous and next waveforms. As a result, the driving-waveform generating circuit becomes a large scale, to cause an increase in the number of driving-waveform signal lines to transmit to a head, and an increase of driver circuits that perform selection of a driving waveform. Such an increase causes an increase in the size of the apparatus and cost.
Moreover, if the processing speed is improved to be high, the residual vibration of not only ejection right before the dot but also of ejections of several dots prior to that can affect it. Therefore, to perform ejection with sufficient accuracy, enormous amounts of driving waveforms are required to be generated, and it is difficult in an actual situation. On the other hand, if the number of driving waveforms is decreased to the realistic number, the correction accuracy is to be not sufficiently high, and it is not sufficient to improve the image quality.
In the following explanation, phenomena of variations in the ejection speed and the ejection droplet amount, and instability of ejection caused by previous ejection are referred to as “temporal interference”, and phenomena of variations in the ejection speed and the ejection droplet amount, and instability of ejection caused by ejection from an adjacent nozzle is referred to as “spatial interference” appropriately.
In either case, it has been difficult to correct the variation in the ejection speed and the ejection droplet amount, and to cause a desired droplet amount of liquid to land at a desired position with high accuracy.